Fertility and reproductive histories of cancer patients, especially of long term survivors of childhood cancer and of men and women who reproduced during cancer therapy are studied for information on the possible mutagenicity and teratogenicity of cancer treatment and to discover hereditary patterns of cancer. Current phases include intensive analysis of data from interviews and medical records of 2,200 survivors of childhood cancer and their sibling controls, to gather information on subsequent morbidity and mortality (especially, additional neoplasms), quality of life, fertility and health of offspring. A second phase is the completion of analysis of a voluntay registry of pregnancies in women with cancer that, to date, shows little, if any teratologic effect, but some excess wastage of pregnancies conceived within 12 months of completing chemotherapy. Finally the principles of mutation epidemiology, in particular, the use of so-called sentinel phenotypes, have been reviewed.